1. Field of the Invention
The present invention relates to a process for the preparation of glass fiber optical waveguides with increased tensile strength by drawing the waveguides from a glass preform with a single-layer or multi-layer additional sheathing of a glass material, with the material of at least the outermost layer in each case having a lower coefficient of thermal expansion as compared with the layer underneath it or with the material of the preform.
2. Description of the Prior Art
Processes of the above-mentioned type are known from DE OS 24 19 76. In the case of optical fibers consisting of a high-refraction core and low-refraction cladding, to improve the mechanical properties, there has already (DE OS 24 19 786) been arranged, over the cladding, a sheath whose coefficient of thermal expansion is lower than that of the cladding and/or lower than that of the combination of the core and cladding. In this manner, in the production of the fiber, a compressive stress is built up in the sheath, which makes the fiber insensitive to tensile and flexural stress.
However, a process is also already known from DE-OS 27 27 054, in which, in order to increase the tensile strength of a glass fiber, there has been additionally applied to the cladding of a fiber preform at least one protective layer of a glass material, which has a lower coefficient of thermal expansion as compared with the adjoining glass material of the fiber preform or fiber. In order to achieve this, a glass based on silicon dioxide which is doped with one or more oxides of the chemical elements lithium, sodium, magnesium, calcium, boron and others is used for the protective layer. The glass preform provided with a protective layer of this type is drawn to a fiber in a fiber drawing machine at the usual drawing temperatures of approximately 2000.degree. C.
In another known glass fiber described in DE OS 28-26 010, in order to increase the tensile strength, the outer sheath consists of at least one layer which, because of its low coefficient of thermal expansion as compared with the cladding, exerts pressure on the fiber-optical structure. Because, in this case, the difference between the two coefficients of thermal expansion should be as large as possible, metal layers are also used to achieve the desired effect, with particular consideration given to the use of aluminum and tin.
However, the mechanical properties of glass fiber optical waveguides prepared in this manner do not meet the requirements which result when optical waveguides of this type, spooled in unwindable lengths, are used for the remote control of instruments and systems.